1. Field of the Invention
The present invention relates to an electrolyte-electrode joined assembly and a method for producing the same, particularly to an electrolyte-electrode joined assembly interposed between a pair of separators and which is suitable for use in a unit cell of a fuel cell and a method for producing the same.
2. Description of the Related Art
A solid oxide fuel cell (hereinafter referred to as an SOFC) contains an electrolyte-electrode joined assembly (for example, a membrane electrode assembly, and hereinafter referred to as an MEA) produced by sandwiching a solid electrolyte between an anode and a cathode. The MEA is interposed between a pair of separators to form a unit cell of the SOFC. The anode is generally composed of a cermet of Ni and Y2O3-stabilized ZrO2 (hereinafter referred to as a YSZ). The solid electrolyte is composed of a material having a high oxide ion (O2−) conductivity, particularly preferably the YSZ.
The cathode is required to have a high ion conductivity, a high electron conductivity, a catalytic activity in an electrode reaction of oxygen thereon (i.e. an oxygen dissociation reaction), a thermodynamic stability (a low reactivity with another substance such as oxygen-containing gas or solid electrolyte), a porosity sufficient for passage of the oxygen-containing gas, a low sinterability in an electric power generation process, and a high mechanical strength. From this viewpoint, perovskite-type composite oxides represented by LaMO3 (M=Mn, Co, Fe) may be selected as a material of the cathode.
Of such perovskite-type composite oxides, LaCoO3, (La,Sr)CoO3 obtained by partially substituting La with Sr (hereinafter referred to as an LSC), or (La,Sr)(Co,Fe)O3 obtained by partially substituting Co with Fe (hereinafter referred to as an LSCF) can be used for the cathode to reduce overvoltage of the SOFC.
La or Sr in the LSC or LSCF is reacted with Zr in the YSZ of the solid electrolyte at high temperature in a firing treatment in production of the MEA or in an operation of the SOFC. Thus, a layer of the reaction product with a high resistance is formed to deteriorate the conductivity.
In order to prevent the deterioration, in Japanese Laid-Open Patent Publication No. 2003-331866, an intermediate layer is formed as a reaction prevention layer. A CeO2-based oxide such as Sm2O3-doped CeO2 is selected as a material of the intermediate layer in this patent document.
However, the CeO2-based oxide has a lower sinterability as compared with the Ni—YSZ in the anode, the YSZ in the solid electrolyte, and the perovskite-type composite oxide in the cathode. In other words, the CeO2-based oxide is a sintering-resistant material.
The intermediate layer composed of the CeO2-based oxide can be sufficiently sintered and densified at a high firing temperature. However, at the high temperature, the CeO2-based oxide in the intermediate layer and the material in the electrolyte are interdiffused, resulting in deterioration of the electric property.
If the firing temperature is lowered in order to prevent the deterioration, the intermediate layer cannot be sufficiently densified. In other words, the resultant intermediate layer is a porous body having a large number of pores. The intermediate layer has open pores on the surface facing the cathode, and the open pores are not in contact with the cathode. Therefore, the contact area between the intermediate layer and the cathode is reduced. In this case, the overvoltage is increased to deteriorate the electric property of the cathode.
European Patent No. 0722193, Japanese Patent Publication No. 07-118327, and Japanese Patent Nos. 2841528 and 2940008 disclose that a certain layer is interposed between the intermediate layer and the anode or cathode.
In the conventional techniques described in European Patent No. 0722193, Japanese Patent Publication No. 07-118327, and Japanese Patent Nos. 2841528 and 2940008, the contact area between the electrolyte and the porous cathode or anode is increased, or an electric path of Ni, Pt, etc. is formed. The above patent documents do not disclose that an intermediate layer is a porous body or that an intermediate layer is interposed between the solid electrolyte and the cathode.
Accordingly, the above problems caused in the formation of the intermediate layer cannot be solved by the conventional techniques described in the above patent documents.